Screen goes black after BIOS splash screen

Nov 3, 2018
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So my PC did work earlier, except that sometimes when I played something(e.g.: Monster Hunter World or Darkest Dungeon) the game crashed and said:
'DXGI DEVICE REMOVED'
Internet saying it may be an overclocked GPU, so lets go into BIOS and see whether it is overclocked.
When I restart I do not come into BIOS.
So I reset my BIOS by removing the battery.

Now the big new problem:
When I start my PC, the BIOS splash screen appears, then the screen goes black, then the '_' icon appears and then it goes completly black.

Whwn I press one of the buttons promts the following happens:

'F9: System Information':
It shows me my system information, processor type, processor speed, RAM amount, BIOS version(F20), BIOS date(01/12/2016, looks like the reset worked) and other stuff
When I press 'Ok', I see the '_' icon again an then the dcreen goes black again.

'F12: Boot Menu':
Shows me my drives to boot from. In this case my SSD where the system is on. When I press on it, the screen goes black, no icon.

'Del':
Screen goes black, '_' icon appears and screen goes black again and stays black.

'End: Q-Flash':
Takes me into the Q flash menu
Exit Q Flash brings up the black screen, then the '_' icon and the black again.

All USB ports work, my leyboard and mouse, too.
I have 2 screens connected to my PC, obly one of them shows the splash screen but both the '_' icon.
I had no bluescreens.

System hardware:
System: Windows 10 64-bit
Motherboard: Gigabyte Ultra Durable X99-UD3
CPU: Intel core i7 5820k
GPU: AMD Radeon R9 280X
Ram: 4 x 4GB
My PC is approximately 3 years old.